The present invention relates to engines and, more particularly, to a fan blade containment structure including means for retaining structural strength in the sealing area of a fan of a fan jet engine after damage due to fan imbalance caused by accidental loss of fan blade fragments.
A fan jet engine includes a high-volume fan at its forward end for forcing ambient air into a flow passage which then splits into a first flow passage entering an axial compressor, combustor and turbine and a second flow passage bypassing the first flow passage for providing direct thrust. Since the fan is at the forward end of the engine, it is the rotating element most at risk of damage in impact with foreign objects such as, for example, birds. A bird strike may damage the fan and, in an extreme case, may dislodge one or more fragments or entire blades of the fan which tend to fly off energetically.
In order to contain such fragments, blade fragment containment structures found in the prior art typically include an annular band of a high strength material such as, for example, steel, surrounding the tips of the fan blades for intercepting such fragments before they can pass out of the engine and cause further damage to the aircraft or surrounding area. Steel is, of course, a heavy material and its use is contrary to the conventional desire for light weight in aircraft structures.
In many types of engines, additional elements such as, for example, inlet cowls, are conventionally supported forward of the fan by the blade containment structure. In the case of a fan structure including a steel armor band, the steel armor band provides substantial strength for supporting such forward structures.
In an attempt to reduce the weight of a blade containment structure, simulations and tests were performed employing a high-strength fabric to replace the steel armor. The high-strength fabric was a fabric commercially known as Kevlar produced by E. I. duPont Nemours & Co. The tests showed that a blanket of Kevlar was successful in retaining fan blade fragments. The test results were reported to NASA in May 1981 in report number NASA CR-165212. In the NASA tests, a layer of honeycomb was provided inside the Kevlar blanket for supporting a sealing strip adjacent the blade tips.
The applicant has discovered that worst-case failure of a fan of a fan jet engine such as, for example, the loss of one blade and one-third of an adjacent blade due to, for example, bird ingestion, seriously unbalances the engine to the extent that the remaining blades of the fan rotate at an increased radius due to an eccentric center of gravity. In this condition, the tips of the remaining fan blades penetrate outward beyond their normal working radius whereby they damage an annular band including the sealing strip and at least the radially inner wall of the honeycomb plus at least a substantial portion of the honeycomb core.
When one wall of a honeycomb structure is completely destroyed, a considerable amount of the strength and stiffness of the honeycomb structure is lost. The remaining strength and stiffness of the damaged structure is little more than the strength and stiffness of the remaining honeycomb wall. Since honeycomb walls are conventionally relatively thin aluminum or composite sheet, the remaining strength and stiffness of the damaged annular region corresponds approximately to the low strength and stiffness of the outer wall of the original honeycomb in this region.
As previously noted, forward structures may depend on the regions surrounding the fan blade tips for their support. Without the strength contributed by the steel band which is eliminated by using a high-strength fabric for the fragment-containment function, the damaged honeycomb structure may have insufficient strength to adequately support the forward structures attached to it and further engine damage may occur.